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Reaction Mechanism and Surface Film Formation of Conversion Materials for Lithium- and Sodium-Ion Batteries: A XPS Case Study on Sputtered Copper Oxide (CuO) Thin Film Model Electrodes Franziska Klein, Ricardo Pinedo, Philipp Hering, Angelika Polity, Jürgen Janek, and Philipp Adelhelm J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10642 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 25, 2015

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The Journal of Physical Chemistry

Reaction mechanism and Surface Film Formation of Conversion Materials for Lithium- and SodiumIon Batteries: A XPS Case Study on Sputtered Copper Oxide (CuO) Thin Film Model Electrodes Franziska Klein1, Ricardo Pinedo1, Philipp Hering2, Angelika Polity2, Jürgen Janek1 and Philipp Adelhelm*3 1

Justus-Liebig-University Giessen, Institute of Physical Chemistry, Heinrich-Buff-Ring 17, 35392 Giessen, Germany 2

Justus-Liebig-University Giessen, 1st Physics Institute, Heinrich-Buff-Ring 16, 35392 Giessen Germany

3

Friedrich-Schiller-University Jena, Institute for Technical Chemistry and Environmental Chemistry, Center for Energy and Environmental Chemistry (CEEC), Philosophenweg 7a, 07743 Jena, Germany

Key words: conversion electrodes, thin film batteries, sodium-ion batteries, copper oxide Abstract Charge storage based on conversion reactions is a promising concept to store electrical energy. Many studies have been devoted to conversion reactions with lithium, however, still many scientific questions remain due to the complexity of the reaction mechanism combined with surface film formation. Replacing lithium by sodium is an attractive approach to widen the scope of conversion reactions and to study whether the increase in ion size changes the reaction mechanisms and benefits or worsens the cell performance. In this study, we use thin film electrodes as additive-free model system to study the conversion reaction of CuO with sodium (CuO/Na) by means of electrochemical methods, microscopy and X-ray photoelectron spectroscopy. The reaction mechanism and film formation are being discussed. Some important differences to the analogue lithium based system (CuO/Li) are found. Whereas CuO has been reported as charge product in CuO/Li cells, charging is incomplete in the case of CuO/Na and only Cu2O is formed. As an important finding, oxygen appears to be redox active and Na2O2 forms during charging from Na2O. Moreover, surface film formation due to electrolyte decomposition is much more severe as compared to CuO/Li. Depth profiling is used to probe the inner composition of the surface film, revealing a much thicker surface film with more inorganic components as compared to the lithium system. It is also found that the surface film disappears to a large extent during charging.

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1. Introduction Electrochemical energy stores play a vital role in our present society and their relevance will further grow as numerous technologies require solutions for the safe, compact and affordable storage of electrical energy on a local scale. Successive improvement of lithium-ion batteries (LIBs) is an important route to fulfil the forthcoming needs, however, also a range of alternative cell systems is currently being studied that aim at overcoming some intrinsic limitations related to conventional lithium-ion intercalation chemistry. Well known examples are the so-called “next-generation”-systems lithium-sulphur (Li/S) and lithiumoxygen (Li/O2). Their theoretical energy densities are several times higher than state-of-the-art Li-ion technologies.1 However, sufficient reversibility under practical conditions has not been achieved to date yet. This is mainly due to the complex chemistry at the positive (sulphur or oxygen) electrode, issues relate to lithium metal as negative electrode and the instability of the available organic electrolytes. Conversion reactions based on the full reduction/oxidation of mostly transition metals – leading to the transfer of several electrons - are another candidate for “next-generation” systems.2 A wide range of compounds such as transition metal oxides, sulphides, fluorides, nitrides and phosphides are being tested for application for anodes as well as cathodes. If commercialized, high capacities and hence high energy density cells could be realized. Nevertheless, also here, the cell chemistry is complex and a number of unsolved challenges such as insufficient and large overpotentials hinder the practical application of these materials for rechargeable room-temperature battery systems.3 Despite the progress of lithium battery technology, lithium is merely on 27th place in the table of abundance of chemical elements and therefore in principle exhaustible.4,5 Whether this will become a limiting factor for LIB technology in the future is, however, a more political question as the resources are unequally distributed around the world. But as the development time of batteries is very long, it is necessary to study potential alternatives as early as possible. Sodium is a conceivable option because of its comparable physical and chemical properties. Additionally, sodium is abundant and therefore an ideal candidate when aiming at low-cost battery cell concepts.1,4,6–14 From a practical perspective, one might speculate, whether replacing lithium by sodium in “next-generation”systems might help to overcome some of the current challenges. Comparing Li/O2 and Na/O2 cells, for example, the sodium cell shows significant advantages, though at the expense of energy density. On the other hand, for low temperature Na/S cells, the performance so far seems much poorer as compared to Li/S cells.1 Scientifically, the question how the replacement of lithium by sodium influences the cell chemistry is quite intriguing and indeed substantial differences should occur also for conversion reactions with transition metals.15 The general conversion reaction to be used in a battery can be written as M X + A ⇄ M + A X with M being a transition metal (or Mg), X a non-metal (i.e. O, S, F, P, N,…) and A an alkali metal (Li or Na), respectively. Commonly, in a conversion reaction a transition metal compound (MX) reacts electrochemically with an alkali metal (A). During discharge, a network of transition metal nanoparticles (M) embedded in a matrix of the alkali compound (AcX) forms. The process is reversed during charging.3,16 A broad range of conversion reactions based on sodium has been studied so far: oxides (CuO 17–23, Co3O4 24–27, NiO 28, Fe3O4 29–32, Fe2O3 33–41, MoO3 42, WO3 43), sulphides (CoS2 44, Co3S4 45, FeS2 46–48, WS2 43,49, MoS2 50–60, Ni3S2 61–64, Cu2S 65), fluorides (CoF2 66), phosphides (NiP3 67), nitrides (VN 68, Ni3N 69), selenides (FeSe 70, MoSe 71, Cu2Se 72) and ternary systems (FeV2S4 73, Mo3Co6S8 74, titanates 75, 2 ACS Paragon Plus Environment

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NiCo2O4 76). Their cell chemistry has been found to be complex in all cases. Compared to the lithium counterparts, the cell voltage is mostly (but not always) lower for the sodium system, the experimentally determined capacities are lower (poor utilization of active material) and the volume expansion is naturally larger due to the larger ion radius of Na+. Differences in the reaction mechanism, however, are much more difficult to unravel, which mostly relates to the nanostructured nature of the electrode after intercalation. Different reaction mechanisms for the conversion reactions of NiO with Li and Na have been recently observed by He et al..77 Another yet (for lithium and sodium-based conversion reactions) poorly understood phenomenon is the surface film formation that appears during the first intercalation. We will use the term solid electrolyte interphase (SEI) for this surface film although also different names such as “gel-like polymeric” film have been used.3 Interestingly, it has been found that the SEI on conversion materials partly disappears during delithiation, the reason though remains speculative.78 Several analytical methods might be applied to study the structural evolution of conversion reactions but a practical hurdle is that the electrodes usually contain larger amounts of carbon conductive additives (typically 15 – 25 wt.%) and binder that are necessary to achieve a reasonable electrode activity. Combined with the rough surface of the particle based electrode, many surface sensitive methods are therefore difficult to apply. Much better suited for analysis are thin film electrodes and are therefore an attractive option for more fundamental studies, as no additives are necessary and their surface is usually reasonably well defined. In this study, we focus on the surface film formation and cell reaction of the conversion reaction between CuO with lithium and sodium, respectively. CuO has been chosen, as it has been already applied in primary cells 79–85and is currently also studied as thin film material for solar cells. Some of the authors have extended experience in preparing such films by sputtering methods.86,87 Different forms of CuO electrodes such as films, nanowires, nanorod arrays and micronanostructured CuO/C spheres have been electrochemically characterized.17–23 It was confirmed by ex-situ XRD, Raman and TEM-measurements that during the first sodiation, the conversion reaction of CuO involves the formation of elemental Cu and Na2O. Cu2O forms as intermediate phase. Lui et al. additionally proposed NaCuO and Na6Cu2O6 as additional discharge products by in-situ TEM experiments.18 During charging, some authors confirmed reappearance of CuO 17,22,23 whereas others exclusively found Cu2O 21 or “6CuO∙Cu2O” 20. Thus, owing to these partly controversial results the cell reaction between CuO and sodium needs thorough enlightenment. It is also important to note that the first cycle is not representative for the subsequent ones as conversion electrodes always undergo an initial activation (see Figure 1). The reversibility of the electrode reaction is therefore better studied after the electrode has been cycled for a few times. In this manuscript, we therefore focus on the 10th rather than the 1st cycle and use XPS to study the electrode reaction. So far, XPS has been applied to study the conversion reaction of CuO with lithium only.88,89 Martin et al. showed for CuO thin film electrodes that Cu2O and Li2O2 form as intermediate and Cu and Li2O as end product during lithiation in the first cycle. Along with this process, electrolyte decomposition lead to an organic/inorganic surface film. The inorganic layer consisted of LiF and resulted from decomposition of the electrolyte conducting salt LiPF6, while the organic, polymeric outer surface resulted from the decomposition of the carbonate solvent. During charging, Cu was only oxidized to Cu2O and so Li2O to Li2O2. The organic part of the surface film disappeared and only LiF remained. Upon subsequent cycling, however, a continuous growth of the polymeric film was observed. Although most studies on conversion have focused on characterizing the reaction mechanism it is clear that understanding the formation of the surface films (SEI) is just as important. Only a stable surface film will enable long cycle life without unwanted excess electrolyte. Only little attention has been paid on this aspect, especially for conversion reactions with sodium. Only a few studies, have 3 ACS Paragon Plus Environment


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been published so far. Komaba et al. showed that in case of hard carbon systems the SEI based on the sodium system contains more inorganic components and is more homogenously structured than in the case of lithium, with inorganic carbonates as main component.90,91 Depending on the system employed, the thickness varies from 5-10 nm (e.g. for hard carbons and Na2Ti3O7) and is therefore thinner than in the lithium counterpart.91,92 For sodium, Edström et al. found a remarkably thick SEI for the conversion system Fe2O3/Na by means of Soft X-Ray Photoelectron Spectroscopy (PES).93 Depth profiling is therefore necessary to study the SEI composition and the buried electrode. This study aims at characterizing the conversion reaction of CuO thin film electrodes with sodium as well as the accompanying surface film formation. Cycling voltammetry and galvanostatic cycling are used as electrochemical methods, SEM is used to study morphological changes. XPS combined with depth profiling is used to study the spatial composition and formation of the SEI layer. The results are compared with the analogous Li-based conversion reaction.


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2. Experimental 2.1. Preparation of free-standing CuO thin film electrodes Cupric oxide (CuO) thin films were prepared by reactive radio frequency sputtering in a mixed (Ar + O2) atmosphere, utilizing a 3’’ metallic copper target. The RF-power applied to the target was set to 50 W, resulting in a density of 1.1 W cm‑2. The working argon and reactive oxygen gas flow rates were kept at 55 and 5 sccm, respectively, adding up to a working pressure of 10-2 mbar. With those parameters the sputtering process yields spatially homogeneous cupric oxide thin films at a depositions rate of 6 nm min-1, where the deposition time was chosen to result in film thicknesses of ~ 200 nm. 2.2. Structural and morphological characterization of CuO thin films The structural properties of the CuO thin films were studied by means of X-ray diffraction (XRD) using a X'Pert Pro (PANalytical) powder X-ray diffractometer (Cu-K source, 40 kV, 40 mA), while their crosssection and surface morphology were investigated by scanning electron microscopy (SEM) using a Merlin high-resolution Schottky field-emission electron microscope (Zeiss SMT). 2.3. Cell assembly and electrochemical measurements The prepared CuO thin film electrodes were assembled in Swagelok-type T-cells (3-electrode configuration) in an Ar-filled glovebox keeping the oxygen and water levels below 1 ppm. CuO thin film electrodes were used as working electrodes while metallic lithium or sodium acting as counter and reference electrodes. Both, Whatman and Celgard separators were placed between counter and working electrodes and were soaked with 80 µL of electrolyte, meaning 0.5 M NaFSI (Sodiumbis(fluorosulfonyl)imide) in EC/DMC (3:7, v/v) (Ethylenecarbonate, Dimethylcarbonate) as sodiumbased electrolyte and LiPF6 in EC/DMC (LP30, battery grade, Merck, < 10 ppm H2O) in the case of Lihalf-cells. The sodium conducting salt was dried at 120 °C under vacuum for 24 hours while the solvents were dried over molecular sieves for at least 72 hours. NaFSI was chosen as conductive salt because we found overall stability and reproducibility to be better for the sodium cell as compared to when other salts such as NaPF6 were used (see Figure S.I.1). Bulk-electrodes were prepared as follows: 65 wt.% active material (CuO or Cu2O, Sigma Aldrich), 25 wt.% conducting agents (Super PLi, Timcal) and 10 wt.% binder (NaCMC, Sigma Aldrich) were mixed to a slurry, stirred for 24 h and casted onto a copper foil (Schlenk) by doctor blading. The prepared electrodes were then dried under vacuum and pressed. Cyclic voltammetry (CV) was performed via a VMP3 equipment (Bio-Logic) by applying a scan rate of 0.05 mV s-1 in the voltage range 0.01 - 3.0 V vs. Na/Na+ or vs. Li/Li+ for both type of half-cells. Galvanostatic measurements were conducted in a Maccor battery cycler (Serie 4000) employing a constant current of 10 µA. Several cut-off voltages were selected in order to investigate the evolution of the surface layer at different states of discharge/charge and cycles. Thus, the defined cut-off potentials during the first and tenth cycle were: 0.01 V (“full discharge”) and 3.0 V (“full charge”). Moreover, 0.4 V vs. Na/Na+ and 0.84 V vs. Li/Li+ cutoff voltages were specifically chosen as a partially first discharged state and 2.2 V vs. Na/Na+ and Li/Li+ as the comparative charged state. It is worth noting that at the partially dis/charged states only Cu2O is expected.94 All electrochemical experiments were carried out at a constant and controlled temperature of 25 °C. 2.4. Morphological characterization Cells were disassembled under Ar-atmosphere and electrodes were rinsed with dried EC/DMC (3:7, v/v) solvent to remove the remaining conducting salt from the surface. Subsequently, samples 5 ACS Paragon Plus Environment


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were dried in vacuum for 3 h under soft conditions (i.e. 65 °C) in order to preserve the morphology. The electrodes were then transferred from the glovebox to the spectrometer and/or SEM by transfer systems under inert argon-atmosphere, avoiding their exposure to atmospheric air. 2.5. X-ray photoelectron spectroscopy (XPS) analysis X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI5000 Versa Probe spectrometer using a monochromatic Al Kα radiation (hv = 1486.6 eV). The peaks were recorded with constant pass energy of 23.5 eV. The diameter of the irradiated area was ~ 200 µm. To minimize charging effects a dual beam charge neutralization system was used. For sputtering and depth profiling an Ar beam with an applied voltage of 1 kV was employed. Photoelectron spectra were fitted with a Casa XPS software with Shirley backgrounds and 100% Gaussian curves. All spectra were charge corrected to a binding energy from hydrocarbons contamination on top of the surface using the C 1s peak at 284.6 eV. During etching the F 1s line at 683.5 eV corresponding to NaF was used for further corrections since the hydrocarbons do not provide a reliable reference line. Quantification was performed on the basis of Scofield’s relative sensitivity factors (R.S.F. values).


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3. Results and discussion 3.1. General aspects of the electrode reaction Conversion reactions proceed in a very particular way which is mainly related to the fact that a complex nanostructure develops during the first lithiation (or sodiation) process. The generally accepted mechanism is sketched in Figure 1 for the lithiation of CuO. The nanocomposite provides intimate contact between the active phases and is an important reason for that the electrode remains electrochemically active during cycling despite the poor conductivity of most phases involved.

Figure 1. Sketch of the generally accepted reaction mechanism of conversion reactions using the example CuO/Li. During the first lithiation, a complex nanostructure develops with copper nanoparticles dispersed in an amorphous matrix of Li2O. This nanocomposite shows partial reversibility in the subsequent cycles. The reaction is accompanied by side reactions with the electrolyte leading to surface film formation.

Although conversion reactions show relatively large overpotentials leading to hysteresis during cycling, it is important to recall the theoretical redox potentials at which the reaction should occur as well as the emerging volume changes. From the different thermodynamically stable phases 95, a range of possible reactions can be considered as shown in Table 1. Starting from CuO, reactions A and G represent the ideal case, i.e. full reduction of Cu (II) and formation of the oxide. Several more reactions (B-F and H-L) might take place when only partial conversion (especially during cycling) takes place. 5

Table 1: Thermodynamic values derived from HSC Chemistry 8.0 (p= 10 Pa, T= 25 °C, bulk thermodynamics, pure phases). Values for theoretical energy densities (wth) are calculated with respect to the total mass of all elements included, i.e. Li, Na, Cu, O. Theoretical capacities (qth) relate to the active electrode mass that is the copper compounds only. Note that due to the low redox potentials (C-J), CuO is a negative electrode material considering application. Lithium

 → Li2O + Cu → Li2O + 2Cu B 2Li + Cu2O  → Li2O2 + 2Cu C 2Li + 2CuO  → Li2O2 + 4Cu D 2Li + 2Cu2O  E 2Li + 4CuO  → Li2O2 + 2Cu2O → Li2O + Cu2O F 2 Li + 2CuO  A

2Li + CuO

Sodium

∆rG° (25°C) / -1 kJ mol − 431.551

E° vs. Li/Li+ 2.24

wth / Wh kg-1 1285

Volume change / % + 74

qth / mAh g-1 674

− 413.302

2.14

731

+ 22

375

− 311.625

0.81

500

+ 35

674

− 275.128

0.71

255

+1

375

− 348.123

0.9

291

+ 34

337

− 449.800

2.33

722

+ 54

337

Volume

qth /

∆rG° (25°C) /

E° vs.

wth /



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1

K

 → Na2O + Cu 2Na + Cu2O  → Na2O + 2Cu 2Na + 2CuO  → Na2O2 + 2Cu 2Na+ 2Cu2O  → Na2O2 + 4Cu 2Na + 4CuO  → Na2O2 +

L

2Cu2O 2Na + 2CuO

G H I J

M N O

2Na + CuO

 → Na2O + Cu2O → NaO2 + 2Cu Na + 2CuO  → NaO2 + 4Cu Na + 2Cu2O  → NaO2 + 2Cu2O Na + 4CuO 

+

kJ mol − 246.705

Na/Na 1.28

Wh kg 546

− 228.456

1.18

− 190.531

0.49

− 154.034 − 227.029

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-1

change / % + 173

mAh g 674

336

+ 74

375

258

+ 67

674

0.40

129

+ 18

375

0.59

173

+ 49

337

− 264.954

1.37

359

+ 103

337

+ 40.433

− 0.10

− 62

+ 55

337

+ 76.930

− 0.20

− 69

+ 12

187

+ 3.935

− 0.01

−3

+ 44

168

-1

The theoretical redox potentials vs. the corresponding alkali metal electrode can be readily derived from the reaction equations. With the exception of reaction A, B and F, reactions C to L can clearly only serve as negative electrodes in practical cells. Voltages are systematically lower in the case of sodium as has been discussed earlier.15 In the case of sodium, the superoxide NaO2 is stable as well next to the oxide and peroxide. However, reactions including NaO2 as discharge product have a positive Gibbs energy and are therefore not expected to occur. Note that the tabulated thermodynamic data for NaO2 might not be completely accurate as the phase is chemically difficult to synthesize. So a slight driving force might not be impossible. Moreover, also nanoscale effects (influence of surface energy, nucleation barriers, etc.) and solid solution behaviour might lead to deviations from bulk thermodynamics. Besides, a range of other kinetic and/or thermodynamic factors related to the ion size can influence the reversibility of the conversion reaction. A common issue for conversion reactions, which strongly affects the degree of reversibility, is the volume expansion as stress and strain can lead to mechanical instabilities of the electrode. Volume expansion is more severe in the case of sodium due to the larger ionic radius compared to lithium15. The smallest volume changes are found for reaction D (+1%) and I (+18%) which might favour reversibility of the electrode reaction as long as other reactions do not take place. The reversibility of the individual reactions between CuO ↔ Cu2O ↔ Cu are still under debate in the case of lithium and are even more unclear in the case of sodium. Usually, the sodium cells show lower practical capacities and a shorter cycle life as the analogue lithium systems. The put forward argument is in most cases the huge volume change during these reactions which, as previously commented, is significantly larger for Na-based reactions, and the result is a decay in the specific capacity. Moreover, the larger ion size of sodium might slow down its diffusion, leading to poorer kinetics. Thus, if conversion reactions seem to be not fully reversible in Li-systems, the possibility that they are not reversible in Na-systems appears more than reasonable at first glance. Nevertheless, whether the peculiar nanostructure that is formed during activation (Figure 1) is reversible or not, is certainly more complex, depends on many factors and on each individual system. A too high diffusion coefficient of one of the reaction partners can also counteract the reversibility, for example.96 Some more direct advantages for the sodium systems are related to the solvation energy, which is smaller for Na+ compared to Li+ as well as the solvation states. This fact has been reported as a reason to explain the relatively lower viscosity and higher conductivity of Na solutions with aprotic solvents.4 Moreover, theoretical studies pointed out that the de-solvation energies of sodium-ion are commonly smaller by ca. 40–70 kJ mol-1, mainly due to the weaker polarization compared to lithiumions.97 This property suggests remarkably lower activation barriers of Na-ion insertion into solid electrodes, as also shown experimentally by Sagane et al.98, and could enable faster charge-discharge kinetics of SIBs compared to LIBs. Another important factor that influences the redox activity of con8 ACS Paragon Plus Environment

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version reactions is the conductivity (ionic and electronic) of the materials. Oxides are mainly insulators (Li2O, Na2O, Li2O2, Na2O2) and NaO2 is likely a poor semiconductor.99–101 CuO and Cu2O are semiconductors with band gaps of Eg(Cu2O) = 2.1 eV, Eg(CuO) = 1.4 eV at room temperature. Nevertheless it is worth noting that conductivity can increase due to non-stoichiometry. The conductivity in CuO thin films can e.g. vary several orders of magnitude, for example.86 Also, the formation of other defects that might enable ionic conduction will differ between lithium and sodium systems.

Overall, the range of possible reactions clearly shows how challenging the correct determination of the electrode reaction becomes. Additionally significant overpotentials and hence a shift of the redox-potentials are expected due to the poor kinetics of solid state phase transitions and the poor conductivity of the materials involved. Redox processes due to electrolyte decomposition further complicate the situation. As a result, the use of cyclic voltammetry to determine the cell reaction is limited as only very broad and ill-defined voltammograms are found (see section 3.3.).

3.2. Characterization of CuO thin film electrodes CuO thin film electrodes were firstly characterized by means of X-ray diffraction (see Fig. 2a). Both, CuO and Cu were identified, the latter being due to the employed Cu substrate. CuO reflections can be indexed on the basis of a monoclinic phase (ICSD #800076) and no impurities were detected. The broad signals indicate nanocrystallinity. The average crystallite size of the copper oxide is around 13 nm as estimated using the Scherrer-equation. Morphological characterization of the surface and cross-section of the CuO thin films was conducted by SEM (Fig. 2b and Fig S.I.2, SI).

Figure 2. a) XRD patterns; b) surface morphology and c) and d) XPS depth profile analyses of the as-deposited CuO thin films.

Homogeneously distributed CuO particles with sizes in the range of 10-80 nm were observed and the layer thickness reaches approximately 200 nm. XPS analyses (Fig. 2c and d) of the Cu 2p core level 9 ACS Paragon Plus Environment


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reveal the presence of several peaks at about 933.6 eV, 940-944 eV and 953 eV. The main Cu 2p3/2 peak at 932.6 eV and the corresponding Cu 2p1/2 peak shifted to 20 eV higher binding energies are due to CuO.102 Satellite peaks in the region 940-944 eV are clearly visible, providing further evidences of the presence of CuO, as this satellite feature is an indication of materials having a partially filled d9 shell configuration in the ground state, such as CuO.103,104 For Cu2O with a completely filled shell (d10), these satellite peaks are absent because screening via a charge transfer into the d states is not allowed.105 It is worth noting that no significant shifts are observed for the Cu 2p spectra and the Cu LMM Auger peak, located at 917.9 eV, during the depth profile analysis confirming that the oxidation state of CuO is kept even after the etching process. 3.3. Cyclic voltammetry Cyclic voltammetry analyses were conducted in order to shed some light on the electrochemical processes of CuO thin film electrodes in both sodium and lithium based half-cells (see Fig. 3). A slow scan rate of 0.05 mV s-1 was selected to separate all electrochemical processes.

Figure 3. Cyclic voltammograms of CuO thin film electrodes for a) Na and b) Li half-cells in the potential range + st th of 0.01 V - 3.0 V vs. A/A (A = Na, Li) obtained during the 1 and 10 cycle.

For the CuO/Na-system, three weak reduction peaks at 1.45 V, 1.08 V and 0.48 V vs. Na/Na+ can be noticed during the 1st cycle. The reduction peak at 1.45 V can be related to the formation of a solid solution, while those at 1.08 V and 0.48 V are ascribed to the Cu(II) to Cu(I) and Cu(I) to metallic Cu reduction processes, respectively.20,23 The intense peak at 0.04 V is usually associated with electrolyte decomposition.91,92,106 Extending the analysis to further cycles reveals that the main differences are found during reduction where the peak at 1.45 V is shifted to higher potentials (1.78 V) and the peaks at 1.08 V and 0.04 V largely disappears. During oxidation (charging) the differences between the 1st and 10th cycle are less obvious. The most well defined peak around 2.0 V remains largely unchanged. Overall, it can be seen that the 1st cycle and 10th cycle (stabilized state) deviate especially with respect to the reduction sweep. This is well in line with the activation of the electrode according to Figure 1. For the CuO/Li-system, the most significant feature during the first cycle is the reduction peak located at 1.2 V, usually associated to the reduction of CuO to Cu2O but also to the decomposition of the electrolyte. This peak is clearly irreversible. No further defined peaks appear during reduction in the first cycle. However, a constant negative current of ─0.015 mA cm-2 flows when approaching 0 V vs. Li/Li+. The current remains negative even when starting the anodic sweep indicating that a slow and continuous degradation mechanism takes place at low potentials. Upon subsequent cycling, the reduction peaks due to CuO → Cu2O and Cu2O → Cu transitions (1.4 and 0.85 V, respectively) and the electrolyte decomposition (0.5 V) become also evident, in good agreement with previous reports.107 Due to the complexity of the electrode reaction, only a few qualitative statements can be concluded from CV experiments: (1) the activation of the electrode during the first lithiation/sodiation can be 10 ACS Paragon Plus Environment

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clearly seen in the voltammogram. (2) In the case of sodium, rapid electrolyte decomposition appears close to 0 V. (3) In the case of lithium, the electrolyte decomposition might be a slower and more continuous process. (4) Peaks associated with electrolyte decomposition decrease upon cycling (although this does not mean that continuous decomposition takes place).

3.4. Reaction mechanism and film formation Due to its high chemical sensitivity, XPS measurements combined with depth profiling provide important information for understanding not only the reaction mechanism but also the SEI formation/dissolution process. Both processes will be examined in the following sections. Results from XPS are further correlated with galvanostatic cycling data. 3.4.1. Reaction mechanism and cycle life The so-called Wagner plot depicts the kinetic energy of the Cu Auger peak against the binding energy of Cu 2p peak (see Figure 4a). Thus, the chemical state of copper refers to the linear relationship between the Auger energy and the photoemission energy.108 In consequence, copper reaction products have been identified by Wagner plots at different states of discharge (0.4 and 0.01 V vs. Na/Na+) and charge (2.2 and 3.0 V vs. Na/Na+) during the first and 10th cycle. The chemical oxidation state of Cu was evaluated after each sputter process in order to examine the reaction products along the depth profile (Figure 4b and Figure S.I.3, SI). Note that the sputter depth cannot be simply calculated from the sputter time. This is because the sputter yield is not known due to the complex microstructure of the electrode. However, from the results discussed below it is clear that the sputter time was long enough to penetrate any surface film. The copper current collector was not reached during sputtering.

Figure 4. a) Wagner plot from data of the CuO/Na-system at different states of discharge and charge and cycles; b) Cu LMM Auger peak after the 10th dis/charge and 720 s sputter time.

Attending to the Wagner plot shown in Figure 4a the following copper reaction products during the first and 10th cycle have been determined.



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Table 2: Copper reaction products in CuO/Na conversion electrodes deduced from the Wagner plot during the 1st and 10th cycle.

Cycle

Discharge/Charge

1st 1st 1st 1st 10th 10th

Discharge Discharge Charge Charge Discharge Charge

Depth of discharge/charge 0.4 V 0.01 V 2.2 V 3.0 V 0.01 V 3.0 V

Copper reaction product Cu2O Cu Cu2O Cu2O Cu Cu2O

During the first cycle CuO is initially reduced to Cu2O (0.4 V vs. Na/Na+) and subsequently to Cu (0.01 V vs. Na/Na+). However, Cu is only re-oxidized to Cu2O during the following charge. The Wagner plot does not show mixed chemical states such as Cu3O4, and CuO seems not to be formed during charge. This phase sequence is maintained during further cycles in good agreement with the doublet (1.78-2.06 V) observed in the CV analysis. Overall, the results clearly evidence reversible conversion between Cu2O and Cu only (reaction H, Table 1) and not between CuO and Cu (reaction G, Table 1). This is in contrast to some earlier studies on CuO/Na-cells where full reversibility was proposed based on CV, ex-situ XRD and Raman measurements.19,22,23 Considering the limitation of reaching only Cu2O during charging, the theoretical reversible capacity of the CuO electrode therefore decreases from 674 mAh g-1 to 337 mAh g-1 (a Cu2O electrode has a theoretical capacity of 375 mAh g-1). It is worth noting, that the behaviour of the present CuO/Na seems different from CuO/Li. Several groups have reported at least partial reoxidation from Cu2O to CuO in the case of lithium, i.e. reaction A occurs at least to some extent.3,78,88,109 Moreover, different type of microstructures are commonly reported in literature 94,110 and could complicate the comparisons as this fact seems to play also an important role due to the favorable accommodation of volume changes in porous spaces during lithium insertion reaction.111 We therefore performed the same XPS depth profiling experiments for the CuO/Li-systems. However, interpretation of the XPS spectra was ambiguous and much less clear as compared to CuO/Na as mixed oxidation states of copper were always found. This can be also taken as hint that reactions A and B occur, which is also consistent with previous works. 112 Figure 5 shows the practically obtainable capacities of CuO electrodes for lithium and sodium cells. The performance of thin film electrodes (estimate for qth,CuO is 75 µAh cm-2) is further compared with conventional electrodes (qth,CuO = 1.2 mAh cm-2 and 674 mAh g-1). Note that the theoretical capacity of sputtered thin film electrodes can only be given as a very rough estimate. Based on the SEM observations, we assume a perfect CuO film with a thickness of 200 nm for the calculation of qth.


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Figure 5. Rate performance of CuO electrodes in lithium and sodium cells with (a) bulk electrodes and (b) thin films. The C-rate is calculated for the ideal case of full conversion of CuO, i.e. 1C corresponds to 674 mA g-1.

All electrodes show a large irreversible capacity in the first cycle, which is expected due to the initial activation of the electrode. Upon further cycling, the capacity values stabilize and decrease with increasing current. Importantly, the capacity values reach their starting values after returning to low current (0.1C). This means that no irreversible mechanical degradation took place in both lithium and sodium cells. It can be also clearly seen that the lithium cells show higher capacities. In the case of conventional electrodes, the capacities are even higher than qth which can be explained by the combination of three factors: i) (seemingly) complete conversion of CuO to Cu; ii) capacitive effects among the copper network that is embedded in a Li2O matrix 113–115 and/or (iii) redox effects related to electrolyte decomposition.116–121 Considering (ii), Ponrouch et al. showed that capacitive effects are probably negligible to the overall capacity.122 Considering (iii), we never observed cell death due to drying out of our cells (neither in the case of lithium nor sodium). We therefore assume that continuous electrolyte decomposition, if it takes place, does not significantly contribute to the decreasing capacity values. Overall, it can be concluded that in the case of CuO/Li, reaction A takes place for the most part. In contrast, the capacities are significantly lower (~ 400 mAh g-1) in the case of CuO/Na. With values around 400 mAh g-1, the capacities are close to what would be expected for partial reoxidation to Cu2O only, i.e. reaction H is the dominant storage mechanism. This correlates well with the results deduced from XPS and CV measurements. Figure 6 shows voltage profiles of CuO thin film electrodes for the 1st and 10th cycle. As expected, very large overpotentials, many steps and a high irreversible capacity are found for the activation cycle due to the complex formation mechanisms of the electrode nanostructure and the surface film. In the case of sodium, the voltage profile becomes smoother upon cycling, whereas more complex mechanisms still take place in the case of lithium. The combined overpotentials are larger in the case of sodium, indicating more sluggish kinetics. Nevertheless it should be noted that the hysteresis might also be partially thermodynamically controlled.123

st

th

Figure 6: Normalized voltage profiles of the CuO thin film electrodes for the 1 and 10 cycle.

3.4.2. Surface film composition, SEI The surface film of conversion electrodes has been already studied a number of times in the case of lithium cells.116–120,124–126 Only a few times, however, the SEI formation and dissolution has been studied.89,118,120,127,128 To the best of our knowledge, no studies on the surface film formation of CuO electrodes in sodium cells have been reported yet. 13 ACS Paragon Plus Environment


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Initial surface film formation in the 1st cycle XPS spectra were recorded after partial discharge (0.4 V), full discharge (0.01 V), partial charge (2.2 V) and full charge (3.0 V). The copper content at these different states of discharge/charge are shown in Figure 7a and is plotted against the etching time.

Figure 7. a) The relative copper amounts calculated from the XPS depth profile in CuO/Na-system, b) cross section SEM images of the CuO thin films after discharge and charge in the first cycle.

Theoretically, the copper content of a CuO electrode is 50 % before discharge and 25 % after full discharge. Partial recharge to Cu2O amounts to a copper content of 33 %. It can be seen that in all cases long sputtering times are needed to even approach these values and the values are significantly too small in the low potential range at 0.01 V (discharge) and 2.2 V (charge). This can only be understood with the formation and dissolution of a surface film (SEI). During discharge, a notable increase in SEI thickness occurs between 0.4 V and 0.01 V. Although the copper content should increase during charging, the values are even a bit lower at 2.2 V vs. Na/Na+ suggesting that the surface film has become even slightly thicker. Above this voltage, the trend is suddenly reverted suggesting partial dissolution of the SEI. The copper content even exceeds the expected value of 33 % which might be a sign for that the current collector has been reached or that preferential sputtering takes place. Formation and partial dissolution of the SEI can be further seen from SEM images (Fig. 7b) of the electrode cross section. A thick surface film of around 6 µm is found after discharge, whereas the thickness after charge is around 1 µm. Considering the thickness of the original CuO layer (200 nm), it is clear that the CuO/Na suffers from a very extensive SEI formation in the first cycle. It must be noted that the average volume expansion for oxides reaches 200 % and 100 % in case of sodium and lithium conversion systems, respectively.15 Thus, in the CuO/Na-system the expansion due to Cu2O formation is 102.9 % and another 74 % during the complete reduction to Cu. Even if the expansion of the electrode only occurs vertically, the maximum thickness of a Cu+Na2O layer would be around 700 nm. So clearly, the increase in electrode thickness is dominated by surface film formation. In line with the XPS results, the SEI layer largely disappears during charging. The thickness of the remaining surface film is roughly around 1 µm. Interestingly, the surface film is much thinner in the case of lithium (see Figure 8) (the composition of the SEI is further discussed below).


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Figure 8. Cross section SEM images of the CuO thin films after the 1st lithiation (discharge) and delithiation (charge).

Surface film in the 10th cycle In order to determine the SEI composition, depth profiling analyses after the 10th discharge and charge had been carried out. XPS analyses of the C 1s and O 1s spectral lines are shown in Figure 9, while those corresponding to Na 1s and F 1s are shown in the supplementary information (Figure S.I. 4 and S.I. 5).

Figure 9. C 1s spectral line for the a) discharged and b) charged states in the 10th cycle as a function of time. The arrow indicates the etching progress. Axes are reversed for better visualization.

C 1s spectra show that carbonates (~ 289 eV) are the main carbon components after both, discharge and charge processes. For the discharged state, the carbonate signal remains constant, even after prolonged etching. In contrast, hydrocarbons and small amounts of alkoxides (284.6 and 286 eV, respectively) are only detectable at the top surface of the SEI after discharge. Overall, the SEI after discharge is mostly composed of inorganic compounds, mainly carbonates, while the organic fraction decreases with depth. This overall composition agrees with that reported previously by Komaba et al., Ponrouch et al., Muñoz-Márquez et al. and Philippe et al..90–93 The residual polymer film after charge is more heterogeneous, with an outer part mainly composed of organic compounds such as hydrocarbons and an inner part mainly consisting of inorganic ones like carbonates but also some esters (at ~ 287.9 eV). Here we remind that due to the absence of carbon additives in the CuO thin film electrode (excluding the unavoidable contamination of the top surface), the carbon signals can be directly ascribed to the decomposition of the electrolyte, or more accurately to the employed organic solvent. The overall carbonate content decreases from 52 % at the end of the discharge to 17 % after charge, pointing out its decomposition during charge, as previously reported in CuO/Li conversion systems by Martin et al..89 Further decomposition products such as NaF are evidenced 15 ACS Paragon Plus Environment


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from the F 1s spectra (Figure S.I. 4). Its presence is related to the decomposition of the conducting salt (NaFSI) and thus, it is homogeneously distributed (i.e. ~4-5 %) along the depth profile after both, discharge and charge processes. Overall, it can be concluded that the increased thickness of the electrode mainly stems from electrolyte decomposition with Na2CO3 and NaF being detected along the whole depth profile.

Figure 10. O 1s spectral line during depth profile for the a) discharge and b) charge state at the 10th cycle as a function of the etching time and their respective Na/ O ratios (c) after discharge and d) after charge).

The O 1s spectra of the electrodes after discharge and charge are shown in Figure 10a,b. In order to provide further evidences about the nature of the reaction products, the Na/O ratios have been calculated after subtracting the NaF and Na2CO3 contributions. These Na/O ratios after 10th discharge and charge are shown in Figure 10c,d. In case of the discharged electrode, together with the main contribution located at 530.5 eV (grey fit in Fig. 10a) and associated to carbonates, especially Na2CO3,92 an extra contribution related to Na2O (526.5 eV, blue fit in Fig. 10a and Na2O reference in Figure S.I.6, S.I.) becomes evident when increasing the etching time over 1440 seconds. It must be noted that this contribution emerges together with the Cu(0) in the Cu 2p region (see Figure S.I. 3a, S.I.). However, the carbonate contribution remains constant with increasing etching time, as it was shown before in the C 1s spectra. After charge, a shoulder with a binding energy at around 529.3 eV (pink fit in Fig. 10b) emerges and grows with increasing etching time due to the formation of Cu2O, in good agreement with the Cu 2p spectrum after charge (see Figure S.I. 3b, S.I.). A further O 1s contribution at 530.5 eV associated to carbonate species can also be observed. With increasing etching time the contribution of carbonates decreases while that one related to Cu2O increases (see also the depth profiles of C 1s in Fig 9 and Cu 2p in Figure S.I. 3). In addition, the small contribution around 526.5 eV representing Na2O disappears after etching for 900 seconds. Thus, a Na/O ratio close to 2 was attained after the discharge (see Fig. 10c), while a Na/O ratio that evolves from 2 to 1 with the etching time was obtained after charge (see Fig. 10d). These data further support the idea that Cu and Na2O are the discharge products, while 16 ACS Paragon Plus Environment

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besides Cu2O, Na2O2 is an additional charge product. In other words, elemental copper is oxidized to Cu2O during charge and additionally Na2O is partially oxidized to Na2O2. This means that, besides Cu and Na, also oxygen is redox active. We emphasize that our proof is only indirect and the following arguments speculative, but redox activity of oxygen might enable alternative reaction mechanisms. Moreover, it is worth noting that Auger peaks present in the O 1s spectra line could result in deviations from ideal Na/O ratios. Therefore, XPS analyses with a dual-anode could provide more accurate evidences. In principle, the following reactions during charging can be formulated (neglecting side reactions with the surface film / electrolyte) as follows: P

2 Cu + Na2O → Cu2O + 2 Na

E0 = 1.2 V (reverse of reaction H)

Q 2 Na2O → Na2O2 + 2 Na

E0 = 1.6 V

R

E0 = 2.4 V

Na2O2 → NaO2 + Na

Reaction P is the cell reaction with Cu2O as charge product. If oxygen becomes redox active and overpotentials for reaction P are large, reaction Q might independently takes place with the result of forming Na2O2. In principle, the formed Na2O2 can further react to form Cu2O or CuO (see reactions J and K). If Na2O2 is inactive, the full oxidation of Cu to CuO is intrinsically limited. Reaction R is unlikely (no proof for NaO2 formation). Clearly, redox activity of oxygen would add to the already complex and poorly understood reaction mechanisms of conversion electrodes. Several reactions might simultaneously compete. Such a behaviour would also provide additional arguments to understand whether the origin of the voltage hysteresis is purely kinetic or also due to thermodynamics. Clearly, more insight is needed. A more thorough analysis of the electrode compositions at different states of discharge and charge has to be part of future studies. 3.4.3. Surface films of CuO/Na and CuO/Li in comparison Finally the SEI after 10th discharge and charge of both CuO/alkali-systems have been compared. For this purpose, the proportions of each carbon species and the total carbon amount have been plotted vs. the etching time in Figure 11. The discharged and charged states are shown in black and blue, respectively, while the different carbonaceous species (hydrocarbons in black, alkoxides in orange, esters in cyan and carbonates in grey) are displayed in the histogram.



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Figure 11. The relative carbon amount of the different carbonaceous species and total carbon amount a) for the 10th discharge in Na- and b) in Li-based systems and c) for the 10th charge in Na- and d) in Li-based systems. Hydrocarbons are denoted in black, C-O bonds in orange, C-O-O-R bonds in cyan, and carbonate species in grey.

Carbon contributions are found within the whole depth profile of CuO/Na half-cells (Fig. 11a,b) after both, discharge and charge processes. In contrast, in the analogous Li-systems, carbon contributions disappear after etching for 720 seconds, as expected from the thicker dimensions of the SEI in the CuO/Na-based systems (~ 6 µm vs. 1.5 µm). These results are in good agreement with previous studies on Fe2O3 conversion-systems,93 although, the thicknesses of the SEIs might have been underestimated for both, sodium-based anode materials and CuO/Li-systems.89,91,92 After discharge the SEI in the CuO/Na-based system is mainly composed of homogeneously distributed inorganic products, such as carbonates, derived from the electrolyte decomposition. In contrast, the SEI of CuO/Li-systems is mainly organic, and mostly composed from hydrocarbons. Their contribution decreases with increasing the sputter time. In turn, the carbonate contribution increases simultaneously. Hence, for CuO/Li a layer-like SEI, with a more organic outer part and a more inorganic inner part is progressively formed which significantly differs from what is found in the case of CuO/Na. These results are supported by the cross section SEM images presented in Figure 8. Although also being quite thick, the surface film after discharge is much thinner as compared to sodium (see Figure 7b). The dynamic formation and decomposition of the surface film on CuO thin films is schematically shown in Figure 12. In the case of sodium, the SEI is much thicker as compared to lithium. The SEI forms during discharge, especially at potentials below 0.4 V vs. Na/Na+ and partially decomposes during charge (> 2.2 V vs. Na/Na+). Furthermore, its composition is more heterogeneous and mainly formed by inorganic constituents.


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Figure 12. Schematic model for SEI-evolution and -composition in CuO/Na and CuO/Li-systems.



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4. Conclusion The conversion reaction of CuO thin films with sodium has been studied. Well-defined and additivefree thin films were chosen as model electrodes to study the reaction mechanism and surface film formation. Electrodes were studied during the first cycle (activation cycle) and the 10th cycle (stabilized state). Considering the electrode reaction, it was found that the CuO/Na-system is only partly reversible. The discharge products are Cu and Na2O, whereas the charge product is Cu2O. This is in contrast to the CuO/Li-system where at least partially CuO reforms during charging. Moreover, it seems that, besides sodium and copper, also oxygen is redox active and Na2O2 forms during charging. The possible appearance of Na2O2 might also help to explain the origin of the voltage hysteresis that is considered to be determined by kinetics and thermodynamics. Morphology and chemical composition of the surface films were studied by SEM and XPS. Surface film formation during first sodiation/lithiation was observed with thickness and chemical composition depending on the alkali metal. For sodium, a very thick SEI (approx. 6 µm!) formed during discharge. The overall composition is fairly homogeneous and inorganic in nature (Na2CO3 and NaF). For lithium, the SEI is around 1 µm thick, structurally more heterogeneous and more organic. During charging, the SEI largely decomposes in the case of CuO/Na and the remaining surface film thickness is around 1 µm. Overall, this study demonstrated that reaction mechanism and surface film formation of conversion electrodes are strongly influenced by the size of the alkali ion. Only few aspects could be clarified so far but there is clearly a lot to be discovered. Understanding of how the size of the alkali ion influences kinetics (diffusion, charge transfer etc.) and thermodynamics (phase stability) as well as surface film formation is a challenging but worthwhile scientific goal. With respect to application, one might hope that some well-known challenges for conversion reactions can be overcome by replacing lithium by sodium, however, no improvement has been demonstrated so far for the here discussed CuO compound as well as for other conversion materials. More systematic studies are needed to clarify the potential for application.


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Author Information Corresponding Author *E-mail: [email protected] Acknowledgement Financial support is gratefully acknowledged from the german research foundation (DFG) within the project “Thermodynamik und Kinetik von Konversionsreaktionen in neuen, natriumbasierten Batteriesystemen“, the state of Hesse (Landes-Offensive zur Entwicklung Wissenschaftlichökonomischer Exzellenz - LOEWE) within the project Store-E (Stoffspeicherung in Grenzflächen). We thank Dr. Luerßen for helpful discussions on data treatment and interpretation. P.A. acknowledges support from the Federal state of Thuringia within the ProExzellenz program.



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